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Metallic states appearing at interfaces between dissimilar insulating oxides exhibit intriguing phenomena such as superconductivity and magnetism. Despite tremendous progress in understanding their origins, very little is known about how to control the conduction pathways and the distribution of charge carriers. Using optical spectroscopic measurements and density-functional theory (DFT) simulations, we examine the effect of SrTiO3 (STO) spacer layer thickness on the optical transparency and carrier distribution in La δ-doped STO superlattices. We experimentally observe that these metallic superlattices remain highly transparent to visible light; a direct consequence of the appropriately large gap between the O 2p and Ti 3d states. In superlattices with relatively thin STO layers, we predict that three-dimensional conduction would occur due to appreciable overlap of quantum mechanical wavefunctions between neighboring δ-doped layers. These results highlight the potential for using oxide heterostructures in optoelectronic devices by providing a unique route for creating novel transparent conducting oxides. [ABSTRACT FROM AUTHOR]

Complex-oxide superlattices (SLs) with atomic-scale periodicity have dynamical properties that are distinct from thin films of uniform composition. The origins of these properties are closely related to the dynamics of polarization domains and to field-driven changes in the symmetries resulting from interfacial coupling between different components. These dynamics are apparent at timescales from a few nanoseconds to several milliseconds in experiments probing the piezoelectricity of a ferroelectric/dielectric BaTiO3(BTO)/CaTiO3 (CTO) SL using time-resolved x-ray microdiffraction. At the 100 ns timescale, the piezoelectric distortion is approximately ten times smaller than in the millisecond regime. This reduced piezoelectricity at short timescales is not observed in previously studied PbTiO3/SrTiO3 SLs or compositionally uniform ferroelectrics such as tetragonal compositions of Pb(Zr, Ti)O3. The unusual behavior of the BTO/CTO SL can be linked to the switching of a nanodomain state into a uniform polarization state or to a field-induced crystallographic symmetry transition. A comparison of the results with the characteristic timescales of these two dynamical phenomena in other complex oxides with different compositions suggests that the phase transition is a more likely possibility. [ABSTRACT FROM AUTHOR]

Theoretical predictions-motivated by recent advances in epitaxial engineering-indicate a wealth of complex behaviour arising in superlattices of perovskite-type metal oxides. These include the enhancement of polarization by strain and the possibility of asymmetric properties in three-component superlattices. Here we fabricate superlattices consisting of barium titanate (BaTiO3), strontium titanate (SrTiO3) and calcium titanate (CaTiO3) with atomic-scale control by high-pressure pulsed laser deposition on conducting, atomically flat strontium ruthenate (SrRuO3) layers. The strain in BaTiO3 layers is fully maintained as long as the BaTiO3 thickness does not exceed the combined thicknesses of the CaTiO3 and SrTiO3 layers. By preserving full strain and combining heterointerfacial couplings, we find an overall 50% enhancement of the superlattice global polarization with respect to similarly grown pure BaTiO3, despite the fact that half the layers in the superlattice are nominally non-ferroelectric. We further show that even superlattices containing only single-unit-cell layers of BaTiO3 in a paraelectric matrix remain ferroelectric. Our data reveal that the specific interface structure and local asymmetries play an unexpected role in the polarization enhancement. [ABSTRACT FROM AUTHOR]

Through the utilization of element-resolved polarized x-ray probes, the electronic and magnetic state of an artificial La2FeCrO6 double perovskite were explored. Applying unit-cell level control of thin film growth on SrTiO3(111), the rock salt double perovskite structure can be created for this system, which does not have an ordered perovskite phase in the bulk. We find that the Fe and Cr are in the proper 3+ valence state, but, contrary to previous studies, the element-resolved magnetic studies find the moments in field are small and show no evidence of sizable magnetic moments in the remanent state. Based on our findings, we argue that the ground state is consistent with canted antiferromagnetic order. [ABSTRACT FROM AUTHOR]

Multiferroic BiCrO3 epitaxial films were grown by pulsed laser deposition on SrTiO3 (001) substrates with SrRuO3 bottom electrodes. The authors found that the films exhibit antiferroelectricity with a characteristic electric field induced ferroelectric phase, as evidenced by double hysteretic behaviors in the electric field dependence of dielectric constants and polarization. The antiferroelectricity is consistent with theoretical predictions that the Bi lone pair induces polarization in bismuth-based perovskites, such as ferroelectric BiMnO3 and BiFeO3. Magnetic measurements revealed weak parasitic ferromagnetism resulting from antiferromagnetic ordering below 140 K. This magnetic transition coincides with a local maximum in the dielectric constants’ temperature dependence. [ABSTRACT FROM AUTHOR]

The effect of epitaxial strain on the charge order (CO) transition in Bi0.4Ca0.6MnO3 films was studied by varying the strain’s strength and symmetry via the use of SrTiO3 and LaAlO3 substrates having different crystallographic orientations. The film on pseudocubic (001) LaAlO3, under symmetric compressive strain, exhibits a clear CO transition. In the film on a (001) SrTiO3 substrate, under symmetric tensile strain, highly segregated line-shaped features in the Bi distribution are seen in Z-contrast scanning transmission microscopy, accompanied by a strongly broadened CO transition. The asymmetric tensile stress on (011) SrTiO3 results in an apparent compressive strain state with a deviation from tetragonality (i.e., γ≠90°), accompanied by the sharpest CO transition. These comparisons illustrate the importance of considering both the strength and symmetry of epitaxial strain. [ABSTRACT FROM AUTHOR]

We report the effect of a definite miscut of yttria-stabilized zirconia (YSZ) (100) single-crystal substrates onto the number of azimuthal domain variants within epitaxial La-substituted Bi4Ti3O12 (BLT) ferroelectric thin films as well as within SrRuO3 electrode layers, both grown on these substrates. YSZ substrates with a miscut angle of 5° were studied, with two different directions of the miscut, viz., YSZ[001] and YSZ[011]. A reduction of the number of azimuthal domain variants by 50% was attained on substrates with a [011]-directed miscut. Due most probably to the reduced number of azimuthal domain boundaries, larger remanent polarization values were attained in BLT films when grown on miscut substrates. [ABSTRACT FROM AUTHOR]

The thermal stability of electrically conducting SrRuO3 thin films grown by pulsed-laser deposition on (001) SrTiO3 substrates has been investigated by atomic force microscopy and reflection high-energy electron diffraction (RHEED) under reducing conditions (25-800 °C in 10-7-10-2 Torr O2). The as-grown SrRuO3 epitaxial films exhibit atomically flat surfaces with single unit-cell steps, even after exposure to air at room temperature. The films remain stable at temperatures as high as 720 °C in moderate oxygen ambients (> 1 mTorr), but higher temperature anneals at lower pressures result in the formation of islands and pits due to the decomposition of SrRuO3. Using in situ RHEED, a temperature and oxygen pressure stability map was determined, consistent with a thermally activated decomposition process having an activation energy of 88 kJ/mol. The results can be used to determine the proper conditions for growth of additional epitaxial oxide layers on high quality electrically conducting SrRuO3. [ABSTRACT FROM AUTHOR]

Observes violet/blue light emitted from certain silicates which may be useful for silicon-based optoelectronics. Production of cerium silicate during a rapid thermal annealing at high temperature from the interface between silicon substrate and cerium oxide film; Cerium silicate experiments through x-ray diffraction, high-resolution transmission electron microscope and Auger depth profiling.

Investigates the memory window of highly c-axis oriented ferroelectric YMnO[sub 3] thin films. Sputtering of YMnO[sub 3] thin films with different oxygen partial pressures from zero percent to 20 percent; Enhancement of the memory window by the c-axis oriented crystallization of the YMnO[sub 3].

To design and discover new materials for next-generation energy materials such as solid-oxide fuel cells (SOFCs), a fundamental understanding of their ionic properties and behaviors is essential. The potential applicability of a material for SOFCs is critically determined by the activation energy barrier of oxygen along various diffusion pathways. In this work, we investigate interstitial-oxygen (Oi) diffusion in brownmillerite oxide SrCoO2.5, employing a first-principles approach. Our calculations indicate highly anisotropic ionic diffusion pathways, which result from its anisotropic crystal structure. The one-dimensional-ordered oxygen vacancy channels are found to provide the easiest diffusion pathway with an activation energy barrier height of 0.62 eV. The directions perpendicular to the vacancy channels have higher energy barriers for Oint diffusion. In addition, we have studied migration barriers for oxygen vacancies that could be present as point defects within the material. This in turn could also facilitate the transport of oxygen. Interestingly, for oxygen vacancies, the lowest barrier height was found to occur within the octahedral layer with an energy of 0.82 eV. Our results imply that interstitial migration would be highly one-dimensional in nature. Oxygen vacancy transport, on the other hand, could preferentially occur in the two-dimensional octahedral plane. © 2014 AIP Publishing LLC. [ABSTRACT FROM AUTHOR]

Strain effects on the electronic properties of (LaTiO3)1/(SrTiO3)N superlattices were investigated using density functional theory. Under biaxial in-plane strain within the range of −5% ≤ ε// ≤ 5%, the dxy orbital electrons are highly localized at the interfaces whereas the dyz and dxz orbital electrons are more distributed in the SrTiO3 (STO) spacer layers. For STO thickness N ≥ 3 unit cells (u.c.), the dxy orbital electrons form two-dimensional (2D) electron gases (2DEGs). The quantized energy levels of the 2DEG are insensitive to the STO spacer thickness, but are strongly dependent on the applied biaxial in-plane strain. As the in-plane strain changes from compressive to tensile, the quantized energy levels of the dxy orbitals decrease thereby creating more states with 2D character. In contrast to the dxy orbital, the dyz and dxz orbitals always have three-dimensional (3D) transport characteristics and their energy levels increase as the strain changes from compressive to tensile. Since the charge densities in the dxy orbital and the dyz and dxz orbitals respond to biaxial in-plane strain in an opposite way, the transport dimensionality of the majority carriers can be controlled between 2D and 3D by applying biaxial in-plane strain. [ABSTRACT FROM AUTHOR]

We have grown non-c-oriented SrBi[sub 2]Ta[sub 2]O[sub 9] (SBT) epitaxial thin films with well-defined (116) orientation by pulsed laser deposition on yttria-stabilized zirconia-buffered (YSZ-buffered) Si(100) substrates covered with electrically conductive (110)-oriented SrRuO[sub 3] (SRO) bottom electrodes. The epitaxial growth of (110)-oriented SRO films on (100)-oriented YSZ on Si(100) was confirmed both by x-ray pole figures and transmission electron microscopy (TEM) analyses showing a diagonal-type rectangle-on-cube epitaxy of SRO on YSZ with respect to the substrate and yielding specific multiple twins which originate from the particular in-plane positioning of SRO on YSZ. Cross-sectional TEM analyses revealed a roof-like morphology at the SBT/SRO interface while the other interfaces are sharp. The ferroelectric measurements of the (116)-oriented SBT films show a remanent polarization (2P[sub r]) of 6.8 μC/cm[sup 2] and a coercive field (2E[sub c]) of 142 kV/cm for a maximum applied electric field of 283 kV/cm. A comparable hysteresis loop recorded from local piezoresponse by an atomic force microscope working in a piezoelectric mode has also been obtained. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The article provides information on the creation of a correlated oxide Dirac semimetal, SrNbO3 thin films, by modifying their symmetry through strain. These strained SrNbO3 films exhibit an ultrahigh mobility, small effective mass, and non-zero Berry phase, reaching the extreme quantum limit. This discovery opens up the possibility of creating novel correlated topological quantum materials.

Raman scattering experiments together with detailed lattice dynamic calculations are performed to elucidate crystallographic and electronic peculiarities of SrCoO3-δ films. We observe that the 85 cm-1 phonon mode involving the rotation of a CoO4 tetrahedron undergoes a huge hardening by 21 cm-1 with decreasing temperature. In addition, new phonon modes appear at 651.5 and 697.6 cm-1. The latter modes are attributed to the Jahn-Teller activated modes. Upon cooling from room temperature, all phonons exhibit an exponential-like increase of intensity with a characteristic energy of about 103-107 K. We attribute this phenomenon to an instability of the CoO4 tetrahedral chain structure, which constitutes a key ingredient to understand the electronic and structural properties of the brownmillerite SrCoO2.5. [ABSTRACT FROM AUTHOR]

The article explores the interfacial tuning of chiral magnetic interactions forlarge topological Hall effects in Lanthanum manganite heterostructures. It mentions the atomistic origin of Dzyaloshinskii-Moriya interaction (DMI) and their relationship to emergent electrodynamic phenomena. It discusses the harnessing interfacial structures to maximize the density of chiral spin textures useful for developing high-density information storage and quantum magnets for quantum information science.

Manipulating the orbital state in a strongly correlated electron system is of fundamental and technological importance for exploring and developing novel electronic phases. Here, we report an unambiguous demonstration of orbital occupancy control between t2g and eg multiplets in quasi-two-dimensional transition metal oxide superlattices (SLs) composed of a Mott insulator LaCoO3 and a band insulator LaAlO3. As the LaCoO3 sublayer thickness approaches its fundamental limit (i.e. one unit-cell-thick), the electronic state of the SLs changed from a Mott insulator, in which both t2g and eg orbitals are partially filled, to a band insulator by completely filling (emptying) the t2g (eg) orbitals. We found the reduction of dimensionality has a profound effect on the electronic structure evolution, which is, whereas, insensitive to the epitaxial strain. The remarkable orbital controllability shown here offers a promising pathway for novel applications such as catalysis and photovoltaics, where the energy of d level is an essential parameter. [ABSTRACT FROM AUTHOR]

Using real-time spectroscopic ellipsometry, we directly observed a reversible lattice and electronic structure evolution in SrCoOx (x = 2.5-3) epitaxial thin films. Drastically different electronic ground states, which are extremely susceptible to the oxygen content x, are found in the two topotactic phases: i.e., the brownmillerite SrCoO2.5 and the perovskite SrCoO3. First-principles calculations confirmed substantial differences in the electronic structure, including a metal-insulator transition, which originate from the modification in the Co valence states and crystallographic structures. More interestingly, the two phases can be reversibly controlled by changing the ambient pressure at greatly reduced temperatures. Our finding provides an important pathway to understanding the novel oxygen-content-dependent phase transition uniquely found in multivalent transition metal oxides. [ABSTRACT FROM AUTHOR]

The crystal and magnetic structures of single-crystalline hexagonal LuFeO3 films have been studied using x-ray, electron, and neutron diffraction methods. The polar structure of these films are found to persist up to 1050 K; and the switchability of the polar behavior is observed at room temperature, indicating ferroelectricity. An antiferromagnetic order was shown to occur below 440 K, followed by a spin reorientation resulting in a weak ferromagnetic order below 130 K. This observation of coexisting multiple ferroic orders demonstrates that hexagonal LuFeO3 films are room-temperature multiferroics. [ABSTRACT FROM AUTHOR]

In the bulk, LaCoO3 (LCO) is a paramagnet, yet the low-temperature ferromagnetism (FM) is observed in tensile strained thin films, and its origin remains unresolved. Here, we quantitatively measured the distribution of atomic density and magnetization in LCO films by polarized neutron reflectometry (PNR) and found that the LCO layers near the heterointerfaces exhibit a reduced magnetization but an enhanced atomic density, whereas the film's interior (i.e., its film bulk) shows the opposite trend. We attribute the nonuniformity to the symmetry mismatch at the interface, which induces a structural distortion related to the ferroelasticity of LCO. This assertion is tested by systematic application of hydrostatic pressure during the PNR experiments. The magnetization can be controlled at a rate of -20.4% per GPa. These results provide unique insights into mechanisms driving FM in strained LCO films while offering a tantalizing observation that tunable deformation of the CoO6 octahedra in combination with the ferroelastic order parameter. [ABSTRACT FROM AUTHOR]

The strong interplay of fundamental order parameters in complex oxides is known to give rise to exotic physical phenomena. The 4d transition-metal oxide SrRhO3 has generated much interest, but advances have been hindered by difficulties in preparing single-crystalline phases. Here we epitaxially stabilize high-quality single-crystalline SrRhO3 films and investigate their structural, electronic, and magnetic properties. We determine that their properties significantly differ from the paramagnetic metallic ground state that governs bulk samples and are strongly related to rotations of RhO6 octahedra. [ABSTRACT FROM AUTHOR]

We use real-time diffuse surface x-ray diffraction to probe the evolution of island size distributions and its effects on surface smoothing in pulsed laser deposition (PLD) of SrTiO3. We show that the island size evolution obeys dynamic scaling and two distinct regimes of island growth kinetics. Our data show that PLD film growth can persist without roughening despite thermally driven Ostwald ripening, the main mechanism for surface smoothing, being shut down. The absence of roughening is concomitant with decreasing island density, contradicting the prevailing view that increasing island density is the key to surface smoothing in PLD. We also report a previously unobserved crossover from diffusion-limited to attachment-limited island growth that reveals the influence of nonequilibrium atomic level surface transport processes on the growth modes in PLD. We show by direct measurements that attachment-limited island growth is the dominant process in PLD that creates step flowlike behavior or quasistep flow as PLD ?self-organizes? local step flow on a length scale consistent with the substrate temperature and PLD parameters. [ABSTRACT FROM AUTHOR]

Ferroelectric control of interfacial magnetism has attracted much attention. However, the coupling of these two functionalities has not been understood well at the atomic scale. The lack of scientific progress is mainly due to the limited characterization methods by which the interface's magnetic properties can be probed at an atomic level. Here, we use polarized neutron reflectometry to probe the evolution of the magnetic moment at interfaces in ferroelectric/strongly correlated oxide [PbZr0.2Ti0.8O3/La0.8Sr0.2MnO3(PZT/LSMO)] heterostructures. We find that the magnetization at the surfaces and interfaces of our LSMO films without PZT are always deteriorated and such magnetic deterioration can be greatly improved by interfacing with a strongly polar PZT film. Magnetoelectric coupling of magnetism and ferroelectric polarization was observed within a couple of nanometers of the interface via an increase in the LSMO surface magnetization to 4.0μB/f.u., a value nearly 70% higher than the surface magnetization of our LSMO film without interfacing with a ferroelectric layer. We attribute this behavior to hole depletion driven by the ferroelectric polarization. These compelling results not only probe the presence of nanoscale magnetic suppression and its control by ferroelectrics, but also emphasize the importance of utilizing probing techniques that can distinguish between bulk and interfacial phenomena. [ABSTRACT FROM AUTHOR]

Pump-probe optical spectroscopy was used to investigate proposed charge-carrier multiplication via impact ionization in the M1 insulating phase of VO2. By comparing the transient reflectivities of the film when pumped at less than and then more than twice the band-gap energy, we observed a larger ultrafast response with the higher energy pump color while the film was still transiently in the insulating phase. We additionally identified multiple time scales within the charge dynamics and analyzed how these changed when the pump and probe wavelengths were varied. This experiment provided evidence that a fast carrier multiplication process, i.e., impact ionization, acts efficiently in this prototypical strongly correlated insulator, as was recently predicted by theoretical calculations. [ABSTRACT FROM AUTHOR]